What reason would a diesel mechanic ever need solder for? Do we even use items such as this? I don't think we would, right?

Used to make field repairs of wiring, it may not be a 10 or 20 level task, but when a blown tire rips out the brake light wiring its your job to make the lights work so the convoy can continue safety to its destination.. put a kit together, small roll of 60/40 solder, small tube of flux, 1 foot of heat shrink tube in each size, 1/8, 1/4, 5/16, 1/2, 3/4", butane soldiering iron.. I have a Weller P2C Butane Soldering Iron kit and Power Probe PPSK Butane Soldering kit for field work and Weller D550-ND gun and Weller WESD51 Digital Soldering Station for shop work.

I do mostly Motorcycle and Auto work, but tons of use on medium diesel vehicles and various non automotive electronics as well.

Fix connections on alternators, starters, relays. It's cheap and you can avoid a much costlier replacement with a bit of solder.

The third shop guys used to use it in some major end item repairs. Also during deployments they guys would sometimes have to repair items that they normally wouldn't in garrison.